This invention relates to an electrical circuit for detecting and holding peak values in a signal supplied thereto. More specifically, the present invention is directed to a peak hold circuit with improved linearity and reduced noise sensitivity.
A conventional peak hold circuit is shown in FIG. 1. An input signal Vinput is supplied to an input terminal 100, and from there to the base of transistor 110. A voltage source 105 is coupled to the collector of transistor 110, and a peak hold capacitor 115 is coupled to the emitter of this transistor. The voltage at the emitter of transistor 110 generally follows the voltage applied to its base. A reset switch 120 and an output terminal 125 are also coupled to the emitter of transistor 110.
Initially, after the reset switch has been opened, the peak hold capacitor 115 stores no charge. So, as the amplitude of Vinput increases, capacitor 115 accumulates charge, and the voltage Vpeak at the emitter of transistor 110 also increases. When the amplitude of Vinput falls below the emitter voltage of transistor 110, the transistor 110 turns off, and, as capacitor 115 lacks a discharge path, the voltage thereacross Vpeak remains unchanged. If the amplitude of Vinput rises above the amplitude Vpeak, transistor 110 turns on and Vpeak increases until a new maximum amplitude is reached. This description excludes the offset caused by the base to emitter voltage Vbe of transistor 110.
The circuit shown in FIG. 1 exhibits two notable problems, high noise sensitivity and poor linearity.
The noise sensitivity problem is illustrated in FIG. 2. As can be seen, if the amplitude of Vinput increases due to a noise burst, then the voltage Vpeak across capacitor 115 reflects this increased amplitude, giving a false impression of the true peak amplitude of Vinput.
The linearity problem is due to the nature of transistor 110, which requires a larger base to emitter voltage Vbe as its collector current increases. To overcome this problem, it is conventional to make the signal swing of Vinput, that is, the magnitude of the change in amplitude, be large so that when Vinput is at a small amplitude, such as 10% of its maximum amplitude, the percent error due to the nonlinear relationship between current and Vbe is within a specified target. However, the requirement for a large signal swing prevents reduction in the voltage source Vcc, and thus impedes cost and size reduction of the peak hold circuit.
A peak hold circuit with improved linearity is shown in FIG. 3, and comprises an input terminal 500 coupled to a positive input of an operational amplifier 510, which has an output coupled to a diode 520; the output of the diode is fed back to a negative input of the amplifier 510, and also is supplied to a peak hold capacitor capacitor 530 and to an output terminal 540.
When a peak value of the input signal is being acquired, that is, as Vinput increases, the output of the amplifier 510 also increases, diode 520 conducts and charge is stored in peak hold capacitor 530. During acquisition, amplifier 510 is in a closed feedback loop so changes in current density of transistor 520 are automatically compensated.
However, the circuit of FIG. 3 has an open loop overshoot problem. Specifically, after a peak value has been acquired, diode 520 stops conducting current so that amplifier 510 operates in an open loop and overshoots, causing a transient error comparable to the error due to noise experienced in the circuit of FIG. 1. To overcome the open loop overshoot problem, it is conventional to provide the amplifier 510 with a fast slew rate, that is, a rapid change in its output in response to large changes in its input, and also a small propagation delay with respect to Vinput, so that the error is not dependent on previous values of Vinput.
Although the circuit of FIG. 3 exhibits improved linearity, it requires a high performance operational amplifier which impedes cost reduction. Furthermore, this circuit has a noise sensitivity problem similar to that of FIG. 1.